Video Transcript
An object travels between point A
and point B, where the points are separated by a distance of two meters. The object travels this distance in
a time interval of three seconds and is moving at a constant velocity throughout the
time interval. Which of the following must be
false? (A) No forces act on the
object. (B) The magnitude of the net force
that acts on the object does not change during the time interval. (C) The speed of the object is
constant. (D) The speed of the object is
zero.
Okay so in this question we have an
object that travels between two points that we’re told are separated by a distance
of two meters. We’re told that the object takes a
time of three seconds to travel this distance and that it moves at a constant
velocity throughout the time interval. We are then given four different
possible statements about the object, and we’re asked to work out which of them must
be false.
In order to answer this question,
we’re going to find it helpful to recall a law known as Newton’s first Law of
motion. This law says that an object at
rest remains at rest and an object with a constant velocity continues to travel at
that velocity unless it’s acted on by an unbalanced force. Newton’s first law means that if
the forces on an object are balanced or there’s no net force on the object, then
that object moves with a constant velocity. If the object isn’t initially
moving, then its velocity remains constant at zero. If it is initially moving, then its
velocity remains constant at a nonzero value. This statement also applies the
other way around.
So if an object is moving with a
constant velocity, whether or not that velocity is zero, then there must be zero net
force acting on the object. Since we’re told in the question
that the object moves at a constant velocity throughout the time interval, this
means we know that throughout this interval there is no net force acting on the
object. With all this in mind, let’s take a
look at each of the four possible statements that we’ve been given.
The first statement in option (A)
is that no forces act on the object. Now, we’ve seen that there must be
no net force acting on the object. And it turns out that there’s two
possible ways this could happen. No net force could either mean that
no forces act on the object at all. Or it could mean that multiple
forces act on the object but that those forces are balanced so that they cancel each
other out. This means that the statement given
in option (A) that no forces act on the object may or may not be true. So we’ve labeled this statement as
could be true.
Next, let’s look at the statement
given in option (B). This says that the magnitude of the
net force that acts on the object does not change during the time interval. We know that this object has a
constant velocity throughout the whole of this interval and that a constant velocity
means no net force. So the net force on the object must
be zero for the whole of the interval. In other words, that net force
remains constant or doesn’t change during the time interval. So the statement in option (B) that
the magnitude of the net force on the object does not change during the time
interval must be a true statement.
Now, let’s look at the statement
given in option (C). This says that the speed of the
object is constant. Now, we know from the question that
the object has a constant velocity. And if the object’s velocity is
constant, then its speed must also be constant. So the statement given in option
(C) must be true.
This leaves us with the statement
in option (D) that the speed of the object is zero. We’re told that the object travels
a distance of two meters in a time of three seconds. And we can recall that the speed of
an object is defined as the distance moved by that object divided by the time taken
to move that distance. In this case, since the distance
moved is not zero, then the speed of the object cannot possibly be zero. And so the statement in option (D)
that the speed of the object is zero must be false.
This means that our answer to the
question of which of the following must be false is that it’s the statement given in
option (D) that the speed of the object is zero.
